Solid state reactions between HTa(PO4)(2).2H(2)O, Zr(HPo4)(2).H2O, VOPO4.2H(2)O and alkali metal chlorides have been studied. The coefficients of proton-alkali metal cation interdiffusion in the reaction products were determined. The increase of the cation radius leads to the growth of diffusion coefficients in HTa(PO4)(2).2H(2)O-MCl and VOPO4.2H(2)O-MCl Systems and their decrease in Zr(HPO4)(2).H2O-MCl system. Estimated interdiffusion activation energies in HTa(PO4)(2).2H(2)O were found to be 80, 50, 30 and 20 kJ/mol for Na, K, Rb, Cs and 120 and 240 kJ/mol for Na+ and K+ ions diffusion in Zr(HPO4)(2).H2O.It was shown that alkali metal cation intercalation into phosphates matrix took place in the course of mechanical treatment. This leads to the dehydration temperature increase and to the decrease in solid state ion exchange process temperature. The mechanism of this process was proposed. (C) 2003 Elsevier B.V. All rights reserved.